nan values in derivatives on pyramid #945
Description
On pyramids, some derivatives of basis functions can be undefined at (0,0,1). For example, the function (xy/(1-z)) is in the polyset, but its derivative (\partial^2/\partial x\partial y) is equal to 1/0 at (0,0,`)
In #938, a test was added that checks on pyramids that if the symbolic value of a derivative is finite, then the Basix evaluation of the derivative is correct.
When the derivative is not finite, it should assert that the Basix evaluation is nan. Currently the Basix evaluations have a value other than nan.